In DICOM, the Pixel Data (7fe0,0010) element is stored with a Value Representation (VR) of "OB" (8 bit unsigned integers) or "OW" (16 bit unsigned integers), i.e. getSint16Array() does not apply. This method is part of DCMTK's dcmdata module, which does not deal with details of the pixel data encoding, such as sign extension.

If you want more sophisticated support for DICOM images, you should consider using DCMTK's dcmimgle/dcmimage module.